Lead-tree solder alloys have become desirable in view of environmental concerns in recent years, and a tin-zinc based solder alloy is known as one of those types.
One application of a solder alloy is to join metal to metal at a relatively low melting point. If the metal to be joined has a composition not contained in the solder alloy, the metal to be joined elutes into a molten solder during a soldering operation, that is, a so-called erosion phenomenon occurs. A copper erosion phenomenon or a silver erosion phenomenon occurs when a target to be joined is copper or silver, respectively, and an intermetallic compound is formed of the eluting metal and a part of the solder alloy composition particularly on a joint boundary and is fixed thereto with solidification. Generation of the intermetallic compound between the metal eluting from the soldering target and the metal contained in the solder alloy is unavoidable in soldering, and the generation itself has no problem. However, if a grain size when the generated intermetallic compound is solidified is relatively large, it causes deterioration to progress on the joint boundary over time. That is, in the intermetallic compound formed of a plurality of types of metal, Kirkendall voids appear due to a difference in diffusion speeds of metal atoms, and there is a concern that the void chronologically develops into a crack due to an external factor such as an external stress, a heat cycle and the like, which might result in breakage of the joint boundary in the end.
If the grain size of the intermetallic compound is large, the difference in the diffusion speeds of the metal atoms is considered to have a great influence and to raise a probability of occurrence of Kirkendall voids. Therefore, even if the tin-zinc-based solder is selected, a solidification grain size of an intermetallic compound on the joint boundary is preferably as small as possible depending on the joint.